Diffraction gratings are utilized in optical systems of various devices, as a spectral element provided with multitudes of parallel periodic structures. In recent years, diffraction gratings are also applied to X-ray imaging devices. Diffraction gratings are roughly classified into transmissive diffraction gratings and reflective diffraction gratings according to diffraction methods. The transmissive diffraction gratings include amplitude-type diffraction gratings (absorptive diffraction gratings) in which light absorption parts are periodically arranged on a substrate for transmitting light, and phase-type diffraction gratings in which parts for shifting the phase of light are periodically arranged on a substrate for transmitting light. In the present specification, absorption means light of an amount larger than 50% of the total light amount is absorbed by a diffraction grating, and transmission means light of an amount larger than 50% of the total light amount is transmitted through a diffraction grating.
Diffraction gratings for near infrared light, visible light, or ultraviolet light can be relatively easily manufactured in view of a point that near infrared light, visible light, and ultraviolet light are sufficiently absorbed by a very thin metal film. For instance, forming a metal film on a substrate by metal vapor deposition on the substrate such as a glass plate, and forming the metal film into a grating pattern enables to manufacture an amplitude-type diffraction grating by a metal grating structure. In an amplitude-type diffraction grating for visible light, in the case where aluminum (Al) is used as metal, forming a metal film having a thickness of about 100 nm for instance is sufficient, because the transmittance of visible light (wavelength in the range of from about 400 nm to about 800 nm) through aluminum is 0.001% or less.
On the other hand, as is well known, generally, X-ray has a property that absorption by matter is very low, and the phase shift is not so large. Even in the case where a diffraction grating for X-ray is manufactured with use of gold, which is a preferable material, it is necessary to form a gold film of about 100 μm in thickness. In the case where periodic structures are formed, with light transmissive parts and light absorption parts/phase shifting parts of a same width and at a pitch of several μm to several ten μm, the ratio (aspect ratio=thickness/width) of thickness to width of the gold part is as high as 5 or more. It is not easy to manufacture a structure having such a high aspect ratio. For instance, patent literature 1 and patent literature 2 are proposed as methods for manufacturing a diffraction grating provided with a structure of such a high aspect ratio.
The diffraction grating manufacturing method disclosed in patent literature 1 is a method for manufacturing a diffraction grating for use in an X-ray Talbot interferometer, and has the following steps. At first, a metal sheet layer is formed on one surface of a glass substrate. Then, patterning is performed by coating an ultraviolet photosensitive resin on the metal sheet layer, and subjecting the ultraviolet photosensitive resin to pattern exposure with use of an optical lithography mask for a phase-type diffraction grating followed by development. Then, an X-ray absorbing metal part is formed on a portion of the metal sheet layer where the ultraviolet photosensitive resin is removed, by a metal plating method. Then, the patterned ultraviolet photosensitive resin, and a portion of the metal sheet layer corresponding to the patterned ultraviolet photosensitive resin are removed. By performing the above operation, a phase-type diffraction grating is manufactured. Then, patterning is performed by coating an ultraviolet photosensitive resin on a surface of the phase-type diffraction grating corresponding to the one surface of the glass substrate, and by subjecting the ultraviolet photosensitive resin to pattern exposure from the other surface of the phase-type diffraction grating with use of the phase-type diffraction grating as an optical lithography mask followed by development. Then, applying a voltage via the metal sheet layer by a metal plating method forms an X-ray absorbing metal part on the X-ray absorption part of the phase-type diffraction grating, on a portion where the ultraviolet photosensitive resin is removed. Thereafter, the aforementioned steps are repeated until the X-ray absorbing metal part has a required thickness, with use of a phase-type diffraction grating having the newly formed X-ray absorbing metal part, as a new optical lithography mask. Thus, an amplitude-type diffraction grating is manufactured.
Further, the method for manufacturing a diffraction grating for an X-ray Talbot interferometer disclosed in patent literature 2 has the following steps:
a groove forming step of forming a groove by alternately repeating an etching step of forming a concave portion by performing preferential reactive ion etching to a silicon substrate with use of gas containing F atoms in an inductively coupled plasma processing apparatus, and a protective film deposition step of depositing a polymer film, as a protective film, on a bottom surface and side wall surfaces of the concave portion by introducing fluorocarbon-based gas in the inductively coupled plasma processing apparatus;
a silicon oxide film forming step of forming an electrically insulating film constituted of a silicon oxide film on a bottom surface and side wall surfaces of the groove by introducing oxygen gas in the inductively coupled plasma processing apparatus;
a silicon exposing step of removing a portion of the electrically insulating film on the bottom surface of the groove, and exposing a silicon portion of the silicon substrate on the bottom surface by performing reactive ion etching with use of gas containing F atoms in the inductively coupled plasma processing apparatus; and
an electroplating step of subjecting the exposed surface of the silicon portion as a seed layer to electroplating to precipitate an X-ray absorbing metal part in the groove.
In the diffracting grating manufacturing method disclosed in patent literature 1, the aforementioned steps are repeated until the X-ray absorbing metal part has a required thickness. This requires a certain time and involves a cumbersome operation.
In the method for manufacturing an X-ray Talbot diffraction grating disclosed in patent literature 2, however, is relatively simple, because the aforementioned steps are not repeated.
However, in the case where an electrically insulating film constituted of a silicon oxide film to be formed on a bottom surface and side wall surfaces of the groove in the silicon oxide film forming step in patent literature 2 is formed by introducing oxygen in an inductively coupled plasma processing apparatus, the thickness of the electrically insulating film is at most about 2 nm, referring to a non-patent literature cited in patent literature 2 i.e. “development of new deep RIE technology using dual side wall protective film” by Junji Ohara and other five persons, Denso Technical Review issued by Denso Corporation in the year of 2000, pp. 45-50, Vol. 5, No. 1, 2000”. It seems to be difficult to further increase the thickness of the film, even if a discharge condition relating to plasma processing, or parameters such as a flow rate of oxygen or a time for irradiating oxygen is changed. Although the aforementioned film may function as a mask for reactive ion etching in the silicon exposing step in patent literature 2, the film has such a small thickness as described above, and does not have a sufficient fineness for actual use. Accordingly, the aforementioned film may not sufficiently function as the electrically insulating film in the electroplating step in patent literature 2, and does not function as an appropriate film in the electroplating step. Since the entirety of a silicon substrate is electrically conductive, X-ray absorbing metal may also grow on the side wall surfaces of the groove in the electroplating step. As a result, hollow portions (voids i.e. portions in which metal is not filled) may be generated in the X-ray absorbing metal part. It is technically difficult to finely fill the groove with the X-ray absorbing metal by electroplating.